The invention relates to a reusable ceramic mold for making foundry castings and more specifically to a ceramic mold having essentially no thermal expansion at the use temperature and which is so configured to prevent sticking or binding of the casting to the mold sections.
The most common foundry mold material for the mass production of articles is sand, compacted around a pattern supported within a mold flask. The pattern is withdrawn, leaving the impressed mold cavity. The sand is usually mixed with an oil or binder to cause the sand particles to cling together. Sand cores may be present within the mold cavity to form recesses in the metal casting. The ordinary sand mold is destroyed by the shakeout procedure required to release the casting.
This art is aware of reusable molds fashioned from a variety of materials, including ceramic and glass-ceramic materials, chosen for their refractory characteristics. Examples of such molds are seen in U.S. Pat. Nos. 4,411,305issued to Beetle, 4,244,551, issued to Terkelsen, and 4,236,568 issued to Larson.
While reusable ceramic and glass-ceramic molds represent a time and cost saving over sand molds, the casting is sometimes difficult to remove from the mold. If the casting has no recesses and is of a completely convex form in nearly all cross sections, i.e., a right circular cylinder or a sphere, then metal shrinkage upon cooling will not inhibit withdrawal from the mold sections. If, on the other hand, the casting has a recessed portion, formed by a projection or core integral with a mold section, the casting will often be difficult to separate from the mold section. This is caused by the greater shrinkage of the metal (3/32 inches per foot or 7.72 millimeters per meter) as compared to the shrinkage of the ceramic, with the somewhat shrunken casting binding on and thus sticking to the projections. In sand molds, metal shrinkage upon cooling does not pose a separation problem because the sand mold is broken apart to obtain the casting.